Radio wave radiating system



Jan. 9 1953 R. REDHEFFER 2,524,343

RADIO WAVE RADIATING SYSTEM Filed June 7, 1945 R-F GENERATOR 5 INVENTORRAYMOND REDHEFFER ATTORNEY Patented Jan. 6, 1953 if sfrAT-as PATENTOFFICE BADIQ WAVE RADIATING. SYSTEM Raymond. Redhefiler; Cambridge,Mass, assignon. by mcsne: assignments, to the. Unitedv St es 01 Americaas. represented bythe. SeQretar-y of War Application June 7, 1945,Serial no. 598,151 3 Claims. (or-2510.43.)

This invention relates in general to electroma netic energy radiatingdevices and more Dar-'- ti'cula'rly to a method for obtaining optimumper formance of such devices.

The importance of matching an antenna for radiation of electromagneticenergy to the transmission means whereby energy is conducted to saidantenna in order to obtain optimum perf-ormance of the system is wellknown in the art. In-many cases it is most desirable to adjust the inputimpedance of the antenna so that standing waves of voltage and currenton the-radio frequency transmission line leadingto the antenna areminimized.

- Among the advantages gained by reduction of standing waves are:greater efiiciency of the transmission line; there is less likelihood ofaltering the R.-F. oscillator output frequency by improper loading; theinput impedance to the line/and hence the power input and output, isless sensitive to small changes of frequency or line length. The last ofthese is the most important. A conventional method of obtaining theproper relation between the antenna impedance and the transmission lineimpedance is to place matching devices such as stub lines, quarter-waveline sections, or line stretchers, at the proper points along the line.The theory and use of these matching devices is well known in the art.and will not be included here. i

: Matching methods of this type, however, tend to be frequency sensitiveand to reduce the. power carrying capacity of the line.

1 Accordingly, it is one object of my invention to provide a method ofmatching an antenna assembly to a radio-frequency transmission line toachieve optimumelectrical characteristics of the transmitting system.

' Another object of my invention is to provide a simple method ofmatching an antenna to a transmission line.

1 Still another object is to provide a method for matching an antenna toa transmission line conveying energy thereto by which proper matchingbetween the two may be obtained in the manufacturing process. I Theseand further objects of) invention will be apparent. to. those skilled inthe art: upon examination of the following specification, claims, anddrawmgs. in which:

- 1 Fig. 1 is a perspective view of a. representative; antenna system towhich my invention is ap plicable; and

" Fig. 2 is. a. vector diagram which illustrates one of the steps in themethod of matching which comprises my invention.

Briefly; the. invention embodies av system forenclosingan antenna. witha protective housing in such 'a, fashion that the reflectioncoefl-icient. introduced by the housingefiectivelycancels that 2 due tothe antenna, thereby eliminating or mini: mizing standing waves in thetransmission 1111 leading to the antenna.

Referring now to Fig. 1, a radio-frequency generator represented byblock5. supplies oscillatory energy to a coaxial transmission line 6.which-feeds an antenna array composed of a. plurality of dipoles l. Acylindrical wave-trans parent housing 9, a portion of which is. showncut away to reveal the antenna structure, sin? rounds the radiatingarray. My invention is particularly adaptable to this type of antennaConstruction. since the radial distance from each dipole! to housing 9is the same. 1

In previous constructions of this nature, it has been necessary toinsert. in transmission line 8 one or more matching devices to permitadiustment. for optimum electrical conditions in line 6;. My invention,however, makes it possible to dispense with such methods.

As has been stated hereinabove, the invention comprises a method ofmatching which can be broken down into five stages or steps.

First, radio-frequency power is fed through transmission line. 6 toantenna array 1 before the latter is placed in a housing. In general,the in put impedance to antenna array 1- willnot be that value which isconducive to. minimum standing waves in line 6. Hence some of the energypropagated toward the antenna in the transmission line will be reflectedback toward the radio-frequency generator. The reflected energy will beequal in amplitude to a constant, [T1, times the amplitude of. theincident'energy and will differ therefrom in. phase by an angle Thesymbol Prep'resents a quantity, hereinafter referred to a the relics!tion coeflicient, such that The. magnitude of the. reflection coemcientis obtained by measuring a maximum voltage pointin the standing wavepattern on the line by any of the,v suitable methods known in the. art,measuring a. minimum voltage point, and taking the ratio, of themaximum. measured voltage to the minimum measured Voltage. This ratiowill hereinafter be desi natedv as the standing wave ratio or p.. ],I[can then be. calculated from the relation 1' [If s (2) The phase angleor, is. unimportant. as will be shown.

The second step is to enclose the antenna array in. a h using of thetype which it is p osed o. use in the complete transmittinginstallation. The addition of a housing will change the antenna inputimpedance as seensirogn the line by reason of the reflections fromthehousing wall. Therefore a new reflection coefiicient, Iz, that of theantenna plus the housing, will exist. The standing wave pattern will ingeneral be different from that measured heretofore and the value of [I2lcan be found from Equation 2 using the new standing wave ratio obtainedfrom measurements similar to those made in step 1.

It is to be expected also that 2 will differ from m. tion of the voltagemaxima and minima in the standing wave pattern on the transmissionline.- The amount of shift is measured and translated into the form ofelectrical degrees by knowledge of the wave-length of energy propagatedon the line. Since the absolute magnitude of 1 and 52 is not important,but rather their relative values, both angles can be measured relativeto any desired reference point. The location on the line of voltagemaxima and minima is a measure of 5, hence if a minimum point when theline. is feeding the antenna array without the housing is taken as areference, the shift in the minimum point due to addition of the housingwill be proportional to 2 1. The equation which may be used forcomputation is where Ad is the shift in the minimum point, x is thewavelength of transmitted energy, and (#2 and qbi are as definedhereinbefore.

Having determined |11|,|12[, and ope-e1), the third step of my inventionis to construct a vector diagram proportional to these quantities. Sucha diagram is shown in Fig. 2, wherein vector A represents thecoeflicient of reflection F1 from the antenna array without a housing,vector B represents the coeflficient of reflection F2 resulting from thehousing and antenna together, and vector C represents the coeflicient ofreflection T3 of the housing alone. Vector C is determined by thevectors A and B since the coefficients of reflection of the antenna andhousing must add vectorially to produce the coefficient of reflection ofthe antenna enclosed by the housing.

It should be emphasized that the coefficient of reflection of thehousing as computed from data on a plain sheet of dielectric material infree space will differ from the apparent coefficient of reflectionintroduced into the transmission lineby placing the housing around theantenna although it will be substantially proportional thereto. The freespace coefficient is customarily used in designing a housing, hence itis necessary to convert the apparent coefficient of reflection into thefree space coefficient. This will be done in a later step.

Construction of the diagram of Fig. 2 shows an angle between vectors Aand C. Clearly, to minimize vector B 0 must first be reduced to zero,putting vector C 180 out of phase with vector A. C then subtracts from Ato give B. When this is accomplished the magnitude of C can be adjustedto approximate that of A, substantially eliminating B. Rotation of C isaccomplished by the fact that a change in housing radius of onewavelength shifts the phase of C relative to A 720, the direction ofrotation depending on the nature of the change in radius. Therefore, thehousing radius may be either increased or decreased, as may bepreferable, to cause vector C to coincide with vector A. The equationrepresenting this relation is This can be detected by a shift in theposiwherein AR is the change in housing radius and 0 and x are asdefined before. Thus the optimum housing radius is determined.

The fourth step is to compute the desired free space reflectioncoefiicient. Vector C is propor tional to the free space reflectioncoeiflcient of the housing used in making measurements and A isproportional to the free space coefficient of the ideal housing since CWill approximately equal A in the final design. The proportionalityfactor is the same in both instances, hence the desired free spacereflection coefficient is found by multiplying the free spacecoeflicient of the test housing by the ratio A/C.

The fifth step is to calculate the thickness dimension of the idealhousing taking into consideration the dielectric constant of thematerial of which it is to be composed and using standard data on thefree space reflection coefflcient of a plain sheet of such material.

If the standing wave ratio existing on the transmission line with thenew housing in place is still higher than is desirable, the process maybe repeated.

To summarize the foregoing, my invention comprises a method of matchingan antenna enclosed in a housing to a transmission line, the methodbeing as follows:

(1) Determine the reflection coefficient of the radiating array alone bymeasurement and Equation 2;

(2) Place a housing of a desired type around the radiating array anddetermine the new reflection coefficient by Equation 2 and the phasedifference between the old and new reflection coefficients by Equation3;

(3) Construct a vector diagram similar to Fig. 2, from the values foundin steps 1 and 2, and use Equation 4 to flnd the optimum housing radius;

(4) Compute the desired housing free space reflection coefficient fromthe free space coefiicient of the housing used for measurements;

(5) Design a housing having the desired free space reflectioncoefficient.

Since the housing itself forms the matching device, a radiating systemconstructed according to the principles of my invention tends to beinsensitive to changes in frequency. This is true because the distanceof the matching device from the radiating elements is an importantfactor in deviations from optimum operating conditions. Tie distance Ywhich represents the distance from a point in the transmission linewhere a conventional matching device would customarily be inserted toone of dipoles in Fig.1 may be several times as great as the distance Xwhich is the distance between radiating elements 1 and housing 9. Hencea change in oscillator froquency will affect the standing wave ratio online 5 and the power output of the antenna less when my invention isused to provide matching.

Also it will be apparent to those skilled in the art that my inventionobviates the need for matching devices in the transmission line itselfand affords the many advantages derived from proper matching of anantenna to a transmission line together with the benefits of protectingthe antenna by a housing.

It is to be emphasized that certain minor deviations may be made fromthe invention and its application as disclosed hereinabove withoutsubstantial loss of its advantages. Hence I claim all such modificationsand adaptations as may fall fairly within the spirit and scope of thehereinafter appended claims.

What I claim is:

1. A wave radiating system comprising a radio wave generator; anantenna; a transmission line connecting the two; said antenna beingmismatched with respect to said generator, whereby a first resultantreflected wave is obtained along said transmission line; and means,including a. substantially wave-transparent dielectric housing at leastpartially enclosing said antenna, for providing a second resultantreflected wave along said transmission line which is substantially equalin amplitude to said first resultant wave and is substantially 180 phasedisplaced therefrom. whereby said second resultant wave substantiallycancels said first resultant wave.

2. A wave guide radiating system according to claim 1, wherein saidhousing alone has a reflection coefiicient substantially equal to thereflection coeflicient of said antenna alone, and wherein said housingis spaced a distance from said antenna which provides a substantially180 phase displacement between said first and second resultant waves.

3. A wave radiating system according to claim 2, wherein said housingconsists of a single cylinder, and wherein said antenna is symmetricallydisposed with respect to the longitudinal axis 01 said cylinder.

RAYMOND REDHEFFER.

REFERENCES CITED The following references are of record in the

